Hey guys looking for some input on the s15 helical diff's before I sink the money into it. I got a s13 with a open diff, I've never had a lsd diff before and I'm wondering how it drives? are there common problems with them?
Also from what I've found this should drop into the open meatball without problem as long as i have the input shafts for it?

The unit itself is pretty bulletproof and helical diffs are extremely ftw for road racing and you'll be able to kick the rear out in the rain but it's not a "limited slip diff" in that by design it will never try to lock the output shafts to a single speed. They're torque biasing; which I'm a little hazy on how it works but basically if one wheel loses traction (inside wheel on hard cornering or one wheel dips into loose gravel or wet leaves or something) it will instantly force the torque you'd otherwise lose to wheelspin to the wheel that has traction. Because there's no clutch plates to bind up it's extremely easy to live with and shouldn't make any more noise or feel dramatically different on your commute than what you're used to now.

However, because how it works is by transferring torque from one wheel to the other, if both wheels lose traction (on ice for example) then there's no torque to transfer and it more or less just acts like an open diff at that point.

Also yeah you need the output shafts and the gearpack itself but also you'd want new bolts for the ring gear, a bit of loctite, new output shaft seals, diff cover seal and maybe new output shaft bearings and also decent torque wrench and a large vice with soft grips to hold the unit while torquing the hardware. There are some shims to locate the unit left or right a few thousandths to ensure proper mesh with the pinion gear so you'd be best served by taking the unit to a shop that can measure the lash and runout and can clean it and build it up for you, but 8/10 it'll probably just drop right into your pumpkin and give you no problems as long as upon reassembly you put the shims back exactly where they came from. The shims are wafer thin disks that go on either side of the housing around the output shafts.

So it almost acts the oposite of an open diff? where an open diff gives power to the loose wheel, this would give power to the traction wheel?
My driving is spirted and hoping to build this car a bit and have the occasional drift or drag days. for that would a 1.5way lsd be better?
I'm finding used s15 helical's for about 600 but the new lsd is about a 1g.
is it worth it to make that jump or would the helical satisfy?
And deff would take it to a shop for final install, cuz i dont have the tool or know how to measure backlash.

Helicals are bad for drifting... in a situation where you're forcing wheel spin they act like an open diff until you grenade them. If you want to know more about them look up "torsen" diffs, they're the same thing and they're super popular with the 4x4 crowd because they offer awesome traction and durability in off-road style situations.

For situations where you want the most grip possible they're the best option, they're also the best option for daily driving as they're the most comfortable/smoothest to drive on after a vlsd. While they'll last much longer than a vlsd they're not going to be as durable as a traditional clutch-based 1.5 or 2-way so they're probably not the best option for drag either.

After a welded diff, 2-ways are probably the best for drift, as well as the best for drag.

2-ways are probably one of the least comfortable to drive on though, in parking lot driving they tend to lock up and cause you to hop around and drag your outside tires like a welded diff, they also clunk around a lot during that type of driving too.

You seem to already know this but just to confirm the S15 helical requires S15 output shafts because the spline count is different than S13/S14 diffs, it will fit in the s13 or s14 pumpkin and bolt up to the normal axles. It's also worth noting that S15 automatics had a 3.69 final drive ratio instead of the normal 4.083 that you get with the 6-speed S15 or the S13/S14.

Personally I'm swapping out the factory VLSD in my LS1 S14 for an S15 HLSD pumpkin with Automatic gears because I'm interested in grip and comfortable driving and the 3.69 gears are better suited to the T-56 transmission.

I also have a Nismo 2-way setup in an S13 pumpkin with Z32 5-bolt output shafts and matching axles. This came out of Jun Fujii's S13 drag car before he went back to Japan. The whole setup is yours for $600 if you're willing to come pick it up. check the link in the sig for pics.

so for enhanced daily driving hlsd is the one for the job?
I might be wrong about this but isn't a 2 way like a user friendly welded diff basically?
I'm still kinda weary about the hlsd holding up to getting loose and dragging on a every other weekend kind of basis twistedsymphony.
I almost like the sound of a regular lsd unit better but I've heard and read a lot of problems with used vlsd's like j30's and hica's diff that are basically shot.
which is why in my mind i keep comming back to somthing more like a 1.5 way lsd new diff?
I guess what its coming down to is does it sound like a hlsd or a 1.5 way lsd gonna be better for me? based of what I've told you guys i plan to do and how i drive.......
I don't mind taking my time to save up and get it done right but I kinda put a cap on this whole diff thing at about $1g, I cant see spending more then that.
thanks for all the input so far guys

Depends on how the cam is ground, i'm sure there are less aggressive 2-ways out there but my old ATS Deftforce was so sensitive that it'd engage and peg-leg down the road if one of my tires were a little low on air pressure.

2-ways are baller asf but they make a daily driver less comfortable to drive daily. Ive heard the same about Kaaz. I witnessed a Nismo 1.5 have a hard time fully locking at a drift event so they're probably made to be more streetable. Helical is best for the street as previously mentioned but you won't be able to hold any sick as hell drifts for very long and twisted seems to think they won't hold up to being abused which sounds reasonable and I can't disprove, but not abused they last forever compared to a VLSD.

Basically it sounds like you're looking for a 1.5 with somewhat forgiving engagement.

So this topic came up in a discussion I had recently, and talking over the benefits/pitfalls of each diff type for most applications we came up with this chart:

obviously there are some other nuances to this, for instance if you have the equipment/ability to change out the spring rates on a 1.5 or 2-way they'd be the better option over hlsd for a professional auto-x/road racing as you can fine tune your diff to the track, however if you're just a weekend warrior the hlsd would likely be the overall better overall option.